1. Field of the Invention
The invention relates to the field of semiconductor lasers, and in particular to tunable semiconductor lasers.
2. Description of the Prior Art
There are a variety of semiconductor laser designs in which the wavelength can be tuned by current injection into a section which has some spectral filtering characteristic. Examples of these are moderately tunable devices such as the distributed Bragg reflector laser, which consists of a gain section, a phase control section and a grating mirror, and widely tunable lasers that employ Vernier effect tuning like the sampled grating distributed Bragg reflector lasers. In all of these devices precise control of the tuning current is required to achieve the desired output wavelength. The amount of current required to achieve a given wavelength can vary with the device temperature and also with aging. Over time as the laser ages changes in the leakage current and the carrier lifetime in the tuning sections cause the wavelength to drift. This variation in the tuning characteristics requires the use of some sort of feedback control system in order for such tunable lasers to be used in applications where precise wavelength control must be maintained such as dense wavelength division multiplexed (WDM) communications systems.
Single wavelength devices such as distributed feedback lasers are also subject to wavelength drift. For these devices simple wavelength lockers are used with temperature or current feedback to control the wavelength of the laser. However these wavelength lockers are only capable of maintaining the laser wavelength on a single channel which makes them useless as multiple-channel tunable lasers.
Tunable semiconductor lasers are important components for next generation dense wavelength division multiplexed fiber optic networks.
What is needed is frequency stable, tunable laser module whose output frequency does not change over time. The device should be integrated on a chip, compact and cost effective. It should have a wide tuning range and be usable in high speed data transmission under direct modulation in multichannel dense WDM networks.
What has been developed is a wavelength monitor based on the transmission response of an optical filter for use in controlling the wavelength of a tunable laser. This monitor can provide feedback on the laser""s wavelength over a wide tuning range enabling it to also lock the device to any given wavelength or channel within its tunable range. It can also be used in combination with conventional external lockers to more precisely tune to a particular wavelength within a given channel.
The invention is also a process for integrating the wavelength monitor directly on chip with a variety of tunable semiconductor lasers. This results in a significantly more compact and cost effective device.
The invention comprises a method for controlling the wavelength of a tunable laser by using a wavelength monitor to measure the output light and provide feedback to a control system. In the preferred embodiment the laser and wavelength monitor are integrated together on a single indium phosphide chip. The technique is useful for control of lasers that are tuned by current injection into sections which have a bandgap energy, which is greater than the lasing wavelength.
The wavelength monitor comprises a wavelength filtering element and a pair of detector elements. In one embodiment these elements are combined by using detectors of different bandgap arranged in series. In others a separate filter with a wavelength dependent transmission function is placed in front of the detectors. In some of these embodiments this separate filter is designed as a wavelength dependent splitter to provide an output beam that translates laterally from one detector to the other as a function of wavelength (much like a prism); in other embodiments the laser output is first split into two beams, and a wavelength dependent transmission filter is placed only in one of these prior to illuminating the detectors. In all cases, taking the difference between in the photocurrents in the two detectors, and normalizing by dividing by the sum, provides a measure of the integrated laser""s wavelength. In all cases, the net filtering function is designed such that the normalized difference current is monotonic and varies from a minimum at one extent of the laser""s tuning range to a maximum at the other extent. This approach can provide channel identification across this entire band of wavelengths, and it can supplement or replace the function of existing external wavelength lockers that only lock within the span of one channel.
More specifically, the invention is an apparatus comprising a tunable laser and a wavelength-dependent optical device to produce two optical signals from light sampled from the laser. The laser is semiconductor laser and wherein the wavelength-dependent optical device is integrally fabricated with the semiconductor laser on a common chip. The two optical signals are wavelength dependent and are distinguished from each other by a different dependence on wavelength. A first and second detector are provided to detect the two optical signals to generate two corresponding electrical detection signals. A processor is coupled to the to first and second detector to generate a control signal from the two corresponding electrical detection signals by which the tunable laser is tuned.
In one embodiment, the wavelength-dependent optical device comprises a two mode interference waveguide. A wavelength-dependent Y-branch splitter is coupled between the two mode interference waveguide and the first and second detector. The first and second detector comprises a segmented detector and further comprises a flared waveguide coupling the two mode interference waveguide and the segmented detector.
In another embodiment, the a wavelength-dependent optical device comprises a wavelength-dependent optical filter. In one embodiment, the wavelength-dependent optical device comprises a wavelength-dependent optical coating. In another embodiment, the a wavelength-dependent optical device comprises a wavelength-dependent optical grating.
In some embodiments, wavelength-dependent optical device comprises one of the two detectors, and is termed a monitoring detector. The monitoring detector has a higher bandgap absorber therein than the other one of the first and second detectors. In one embodiment, the monitoring detector is coupled inline between the tunable laser and the other one of the first and second detectors.
In still another embodiment the apparatus further comprises an attenuator and a filter. The monitoring detector is coupled inline with the other one of the first and second detectors which is coupled to the tunable laser. The attenuator and the filter are coupled in line between the monitoring detector and the other one of the first and second detectors.
In still another embodiment, the apparatus further comprises a splitter. The splitter directs a first portion of light from the laser to the first detector and a second portion of light from the laser to the coating. The second detector detects light reflected from the coating.
In one embodiment, the apparatus further comprises a diffractor. The diffractor directs a first portion of light from the laser into the coating such that the first portion is totally internally reflected and a second portion of light from the laser into the coating such that the second portion of light from the laser is reflected through the coating to the second detector.
The invention is also defined as a method of operating a tunable laser comprising the steps of sampling light from the laser and producing two optical signals from light sampled from the laser. The two optical signals are wavelength dependent and distinguished from each other by a different dependence on wavelength. The two optical signals are detected to generate two corresponding electrical detection signals. A wavelength dependent control signal is generated from the two corresponding electrical detection signals. The control signal is fed back to the tunable laser to control wavelength of light generated by the laser.
Again in one embodiment the step of producing two optical signals is performed by creating a wavelength dependent, beat pattern between two modes of light.
In another embodiment the step of producing two optical signals is performed by monotonically filtering the light as a function of wavelength. For example, the step of producing two optical signals is performed by monotonically filtering the light as a function of wavelength by transmission through a grating or by reflection from a grating. In the latter case, the two optical signals are produced by monotonically filtering the light as a function of wavelength by reflecting the light off a back facet of the tunable laser through a wavelength dependent coating disposed on the back facet.
The tunable laser has a tuning range, and the degree of filtering of the intensity of the light is varied between extremums corresponding to the tuning range of the laser.
The step of creating a wavelength dependent, beat pattern between two modes of light is performed for several beat lengths of the two modes of light and then comprises splitting the two modes light into first and second paths and thereafter detecting the two modes of light in the first and second paths. The step of splitting the sampled light into a first and second path comprises splitting the two modes of light into the first and second path with a wavelength dependent splitting ratio.
The method further comprises determining total power output from the laser by summing the two corresponding electrical detection signals. The wavelength dependent control signal is generated from a ratio of intensity of the light detected corresponding to the two optical signals by normalizing the control signal with the total power output.
In one embodiment, the step of splitting the sampled light comprises transmitting the two modes of light to two segmented detectors. Detection of the two optical signals comprises detecting the light in the two segmented detectors to provide two complementary sinusoidal detection signals. The step of generating a wavelength dependent control signal comprises generating the control signal as a ratio between the difference of the two complementary sinusoidal detection signals and the sum of the two complementary sinusoidal detection signals.
In some embodiments the steps of producing and detecting the two optical signals comprises detecting the light with a first detector with an absorber having a bandgap energy low enough to provide substantially full absorption of the light, and detecting the light with a second detector with an absorber having a bandgap energy slightly greater than the shortest wavelength of the light so that absorption in the second detector varies with wavelength of the light.
In another embodiment the step of producing two optical signals comprises diffracting the light back onto a back facet of the laser so that a portion is totally internally reflected and a portion reflected back from a wavelength dependent coating disposed on the back facet.
In another embodiment the step of producing two optical signals comprises splitting the light into two detectors in which at least one of the detectors detects the light in a wavelength dependent manner. For example, the step of producing two optical signals comprises transmitting the light through a first detector into a second detector in which at least one of the detectors detects the light in a wavelength dependent manner. The step of producing two optical signals comprises transmitting the light first through the first detector and then into the second detector in which at least the first detector detects the light in a wavelength dependent manner. Alternatively, the step of producing two optical signals comprises transmitting the light first through the first detector, then through an attenuator and filter in optical series, and then into the second detector in which at least the second detector detects the light in a wavelength dependent manner.
The invention can also be characterized as a method of operating a tunable laser comprising the steps of sampling light from the laser, and generating two signals from the light sampled from the laser, which two signals are wavelength dependent and distinguished from each other by a different dependence on wavelength. Two detected signals corresponding to the two signals are generated. The two detected signals are combined to produce a wavelength dependent control signal. The laser is tuned by feeding back the wavelength dependent control signal to the tunable laser.
Although the method has been described above for the purposes of grammatical ease in terms of steps, it is to be expressly understood, that the claims are not to be limited by xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d restrictions based on a construction under 35 USC 112. The invention having been summarized can be better visualized by turning to the following drawings wherein like elements are referenced by like numerals.